Art of dehydrating and impregnating wood



HOURS URYING TIME TO OBTAIN 30% RESIDUAL MOISTURE IN WOOD Feb. 3, 1948.

M. s. HUDSON 2,435,219

ART OF DEHYDRATING AND IMPREGNATING WOOD Filed Sept. 17. 1947 PER CENTBY WEIGHT OF ORGANIC VAPOR FOR DRYING CHAMBER ATMOSPHERE Inventor MON/E.s. HUDSON B award/$1 ,4 il'orn ega- Patented Feb. 3,1948

UNITED STATES PATENT OFFICE ART OF DEHYDRATING AND IMI'BEG NATING WOODMonie S. Hudson, Spartanburg, S. 0. Application September 1'7, 1941',Serial No. 174,639 '1 Claims. (01. 117-57) This application is acontinuation-in-part of my copending application Ser. No. 324,893, fliedMarch 19, 1940, now U. S. Patent No. 2,273,039, granted February 1'7,1942, and my copending application Ser. No. 431,176, filed February 16,1942, and allowed April 4, 1947.

In my above Patent 2,273,039 I have disclosed and claimed a process fordrying wood with heated organic vapors, the process being carried out ina closed chamber in which the atmosphere is substantially saturated withorganic vapor; that is, as rapidly as the moisture is liberated from thewood during the drying process it is removed from the chamber so thatthe'atmosphere around the wood is substantially saturated with organicvapor. 'The temperature in the drying chamber is maintained above 212 F.

and up to about 450 F. That method of drying wood with an atmospherecomposed substantially entirely of the organic medium enabled wood andwood products tobe dried commercially in a matter of hours instead ofdays as was required in prior conventional practice. One practicalobjection to that process is the relatively high cost of the fuel forvaporizing the organic compounds and the cost-of handling the relativeiylarge amount of organic vapor condensate produced during the dryingoperation. This is especially true where concentrations of about99%-400% organic vapor are attempted to be maintained.

In accordance with the present invention, I have discovered that asubstantially saturated atmosphere of organic vapor in the dryingchamber, as disclosed in my above patent, is not necessary and that,within certain limits, moisture vapor may be permitted in the dryingchamber, and with substantially less fuel and handling costs. Animportant factor is to prevent too long a drying time which wouldcausehydrolysis of the wood in the presence of the moisture vapor and wouldalso involve excessive costs of operation of the drying cylinder forsuch long period of time.

I have found that the extreme limit of water vapor is around 50% (theother 50% being organic vapor) and that operation at or above thatpercentage of water vapor is unreliable and protracts the dryingoperation much too long; and that a good reliable and economicalpercentag of water vapor is approximately 35%. At this percentage theorganic vapor content of the atmosphere in the drying chamber would beapproximately 65%.

This organic vapor percentage may be increased (with correspondingdecrease in the water vapor content) up to 100%, theoretically. but Ihave found from actual experience in the commercial operation of theprocess that there is no real practical advantage in increasing theorganic vapor content above about 90%; and that increase in the dryingtime. Accordingly a good commercially advantageous range is about toorganic vapo with a corresponding moisture vapor range 01' about Thesingle figure in the drawing is a graphic illustration of therelationship between the drying atmosphere concentration and varyingtime, characteristic of the drying process of this invention.

The curve shown in the drawing is based upon data from various teststhat I have made but is to be understood as representative onlygenerally of the relationship between drying vapor con--' centration andhours drying time for my process. It would naturally vary some with eachparticular type and size of timber or other wood products being dried.The very steep part oi the curve corresponding to about 50% and less oforganic vapor shows that a slight decrease in percentage of organicvapor causes a very large it is not practical to operate underconditions that fall under that portion of the curve.

At the other extreme limit, that is, approximately organic vapor, thecurve is flat and from the standpoint of time of operation, it issatisfactory, in fact, his optimum. However, practical experience hasshown that the cost of operation rises extremely rapidly as a saturatedorganic vapor atmosphere is approached and that above 99% organic vaporthe cost goes infinitely high.

It is therefore apparent "from the above facts and from the curve in thedrawing that the range of about 65% to 90% is a good, safe, andcommercially advantageous range in which to operate, and that theorganic vapor percentage may be increased or decreasedto some extent oneither side of thisrange but without any great benefit. As compared withthe saturated atmosphere process of my above patent, the process of thepresent application, especially when operated in the approximate 65% to90% range, results in a very substantial reduction in the fuel cost, forheating the organic medium, and a substantial reduction in the amountoflorganic condensate which must be handled and reclaimed during thedrying operation.

In carrying out the process of my invention I use any suitable dryingapparatus having a closed chamber and which is preferably maintained atsubstantially atmospheric pressure, although in some cases a reducedpressure or vacuum may be employed. An example of a suita- 35% to 10%.The higher the percentage oi organic vapor the raster of approximately65% to which was filed concurrently with my above identified applicationSer. No. 431,176, r which the present application is acontinuation-in-part; the said application Ser. No. 431,177 having beenallowed on May 7, 1947. For the operation or my process with thatequipment, vapor-pressuretemperature relations or the organic materialto be used in the process are determined on samples, and a graph made upby using Dalton's law calculations showing the relation of percentage ofweight of organic material in the vapor phase, (water being the Bcomponent) to temperature. A temperature corresponding to aconcentration oi about 65%, for example, of organic material in thevapor phase is selected from this chart. This temperature is then set ona thermostatic-recording controller whose actuating bulb is situated inthe drying chamber of the drying apparatus. 1

This controlling device operates a suitable mechanism for varying theheat. input to the evaporator which might be part of or a separatevessel from the drying chamber. In this evaporator, the organic dryingmedium is maintained at its boiling point, and any impulse 'set up bythe thermostatic controller increases or decreases the heat input, whichmodifies the boil-up of organic drying material into the cylinder, timemaintaining the concentration or organicmaterial always within thedesired critical limit.

The wood or wood products to be dried are placed in the drying chamber,and the temperature used is one which will assure in'the apparatusthroughout the actual drying period (exelusive. oi the short start-upperiod), an organic vapor concentration within 1 the range ofapproximately 65% to 90%. Toinsure an accurate check on this, duringdrying, I employ suitable meters and a continuously separating device bywhich the proportion of organic material to water in the condensate maybe measured and held at the desired percentage by controlling the. heatinput to the system.

Suitable organic drying media for use in the present invention arelisted below, and some of these are also excellent preserving materialsfor wood and wood products.- Furthermore, where impregnationwith'preservativedrying media is desired, this may be accomplishedsubstantially with the drying, so that drying and preserving is carriedout as a single step, thus eliminating the subsequent pressureimpregnation cycle which is necessary if a non-preserving drying agentis used.

Preferred examples of suitable organic drying media are:

Distillates of petroleum and coal tar and distillates of wood and woodtar.

Alcohols, such as butyl, isobutyl, amyl, diethylene glycol, or glycerol.

Aldehydes, such as butyric or furfural.

Amines, such as aniline or quinoline.

-Esters, such as glycol mono-ethyl-ether acetate orbutyl-oramylacetates.

Ethers, such as diamyl ether.

Halides, such as dichloro phenol, tetrachlorphenol, pentachlorphenol andparadichlorobenzene.

Nitro compounds, such as nitrobenzene or orthonitro-chlorobenzene.

Hydrocarbons. such as mesitylene, xylene, toluene, benzene, dipentene,or diphenyl.

Phenols. creosols, or xylenols.

For simultaneous numerous or the above compounds or mixtures thereof aresatisfactory, e. g., tetrachlorphenol alone or in solution in apetroleum distillate. Compounds having boiling points at temperatureshigher than can be used in contact with the wood may be utilizedin-solution in solvents that ab-v normally raise their vapor pressuresso that they can be distilled at lower temperatures. 1

Certain oi the high melting point compounds will be found to possessvapor pressures that are too low for satisfactory operation as dryingand impregnating media at atmospheric pressure, that is, too high atemperature would be required. to volatilize appreciable amounts atatmospheric pressure even though a solvent which raises the vaporpressure of the solute abnormally, is used. This difficulty may be,easily overcome by employing a liquid solvent having a boiling pointnear that or the toxic solute and impressing vacuum on the system tolower the boiling point'of the mixture.

The term wood and wood products" as used herein is intended tocomprehend such materials as green cross-ties, structural timbers,boards and other 7 acterized by commercial subjecting the wood to forms,pulps, wood flour of various kinds. wood molding powders, etc.

I claim: 1. A rapid, high temperature method 01' dehydrating wood in aclosed space, which is chardrying oi' the wood in a matter of hoursinstead of days, comprising composition containing not substantiallyless than about 65% by weight oi. an inert organic vapor but less than asaturated atmosphere of said vapor, and an appreciable amount or watervapor not in excess of about; 35% by weight, said organic vapor being ofa type suitable for permeatingthe wood and replacing the water in saidwood and having no adverse effects on the wood, and said vaporcomposition being heated to and maintained during drying at a hightemperature substantially above the vaporization temperature of themoisture in the wood so that the moisture is flashed ofl as vapor,progressively displacing the liberated moisture vapor so that the watervapor content of said drying vapor composition does not substantiallyexceed 35% by weight at any time during the drying treatment, andcontinuing this treatment until the desired degree 01' dryness of thewood is obtained.

2. A rapid, hi h temperature method or dehydrating wood in a closedspace, which is characterized by commercial drying oi the wood in amatter of hours instead of days, comprising subjecting the wood to ahighly heated vapor composition containing approximately 65% to 90% byweight of an inert organic drying vapor and approximately 10% to 35% byweight of water vapor, said organic vapor being of a type suitable forpermeating the wood and replacing the water in said wood and having noadverse eflects on the wood, and said vapor composition being heated toand maintained during drying at a high temperature substantially abovethe 4 drying and impregnating,

or partially seasoned poles, pillngs,

a highly heated vapor desired degree of dryness or the wood is obtained.

3. A rapid, high temperature method of dehydrating'wood in a,closedspace, which is characterized by commercial drying of the wood in amatter of hours instead of, days, comprising subjecting the wood to ahighly heated vapor composition maintained at a temperaturesubstantially within the range of about 212 F. to 450 F. and containingnot substantially less than about 65% by weight of an inert organicvapor but less than a saturated atmosphere of said vapor, and anappreciable amount of water vapor not in excess of about 35% by weight,said organic vapor being of a type suitable for permeating the wood andreplacing the water in said wood and having no adverse eflects on thewood. and said vapor composition being heated to and maintained duringdrying at a temperature substantially above the vaporization temperatureof the moisture in the wood so that the moisture is flashed of! asvapor, progressively displacing the liberated moisture vapor so that thewater vapor content of said drying vapor composition does notsubstantially exceed 35% at any time during the drying. treatment, andcontinuing this treatment until the desired degree of dryness of thewood is obtained.

4. A rapid, hightemperature method of dehydrating wood in a closedspace, which is characterized by commercial drying of the wood in amatter of hours instead of days, comprising subjecting the wood to ahighly heated vapor composition containing not substantially less thanabout 65% by weight of an inert vapor of a wood preservative organiccompound but less than a saturated atmosphere of said vapor, and anappreciable amount of water vapor not in excess of about 35% by weight,said organic vapor being of a type suitable for permeating the wood andreplacing the water in said wood and having no adverse efiects on thewood, and said vapor composition being heated to and maintained duringdrying at a high temperature substantially above the vaporizationtemperature of the moisture in the wood so that the moisture is flashedoil as vapor, progressively displacing the liberated moisture vapor sothat the water vapor content of said drying vapor composition does notsubstantially exceed 35% at any time during the drying treatment, andcontinuing this treatment until the desired degree of dryness or thewood is obtained and the wood is impregnated after the drying treatmentwith said wood preservative organic compound.

5. A rapid, high temperature method of dehydrating wood in a closedspace, which is characterized by commercial drying of the wood in amatter of hours instead of days, comprising, maintaining a reducedpressure in said closed space, subjecting the wood to a highly heatedvapor composition containing not substantially less than about 65% byweight of an inert organic vapor but less than a saturated atmosphere ofsaid vapor, and an appreciable amount oi water vapor not in excess ofabout 35% by weight, said organic vapor being of a type suitable forpermeating the wood and replacing the water in said wood and having noadverse eflects on the wood, and said vapor composition being heated toand maintained during drying at a high temperature substantially abovethe vaporization temperature of the moisture in the wood so that themoisture is flashed of! as vapor, progressively displacing the liberatedmoisture vapor so, that the water vapor content of said drying vaporcomposition does not substantially exceed by weight at any time duringthe drying treatment, and continuing this treatment until the desireddegree of dryness oi the wood is obtained.

6. A rapid, high temperature method of de- I hydrating wood in a closedspace, which is char,- acterized by commercial drying of the wood in avmatter of hours instead of days, comprising subjecting the wood to ahighly heated vapor composition containing not substantially less thanabout 65% by weight of an inert organic vapor but less than a saturatedatmosphere of said vapor, and an appreciable amount of water vapor notin excess of about 35% by weight, said organic vapor containing a toxicpreservative I compound, and being of a type suitable for permeating thewood and replacing the water in said wood and having no adverse effectson the wood, and said vapor composition being heated to and maintainedduring drying at a high temerature substantially above the vaporizationtemperature of the moisture in the wood so that the moisture is flashedoi! as vapor, progressively displacing the liberated moisture vapor sothat the water vapor content of said drying vapor composition does notsubstantially exceed 35% by weight at any time during the dryingtreatment, and continuing this treatment until the desired degree ofdryness of the wood is obtained.

'7. A rapid, high temperature method of dehydrating wood in a closedspace, which is characterized by commercial drying of the wood in amatter of hours instead or days, comprising subjecting the wood to ahighly heated vapor composition containing not substantially less thanabout 65% by weight of an inert organic vapor but less than a saturatedatmosphere of said vapor, and an appreciable amount of .water vapor notin excess of about 35% by weight, said organic vapor being obtained byheating an organic compound selected from the group consisting ofdistillates of petroleum, coal tar, wood and wood tar, the said-organicvapor being of a type suitable for permeating the wood and replacing thewater in said wood and having no adverse effects on the wood, and saidvapor composition being heated to and maintained during drying at a hightemperature substantially above the vaporization temperature of themoisture in the wood so that the moisture is flashed ofl the wood isobtained.

' MONTE B. HUDSON.

Certificate of Correction Patent No. 2,435,219. r Februery3,1948.

MONIE S. HUDSON It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows: Column 3, line 51, for the word substantially readsimultaneously; and that the said Letters Patent should be read withthis correction therein that the same may conform to the record of thecase in the Patent Office.

Signed and sealed this 6th day of April, A. D. 1948.

THOMAS F. MURPHY,

Assistant Uommissioner of Patents.

